Enzymes that catalyze the formation of isoprenoids are of interest as drug targets. The Rohmer pathway, also known as the methyl erythritol phosphate or non-mevalonate pathway, is responsible for isoprenoid biosynthesis in most pathogenic bacteria and in malaria parasites, such as Plasmodium faciparum (Rohmer et al., Lipids 2008, 43(12), 1095; Wiesner and Jomaa, Current Drug Targets 2007, 8(1), 3). Enzymes found in the pathway are potentially important as anti-infective drug targets because isoprenoids are essential for survival of these microorganisms and because the non-mevalonate pathway is absent in humans (Williams and McCammon, Chem. Biol. Drug Des. 2009, 73(1), 26; de Ruyck and Wouters, Curr. Protein Pept. Sci. 2008, 9(2), 117).
The structures and mechanisms of action of six of the eight enzymes present in the pathway are now known. Fosmidomycin, which inhibits the second enzyme in the pathway, has shown promising results for treating malaria (Jomaa et al., Science 1999, 285, (5433), 1573; Borrmann et al., Antimicrobial Agents and Chemotherapy 2006, 50(8), 2713). An inhibitor of one of the six known enzymes, deoxyxylulose-5-phosphate reductoisomerase, has been used clinically to treat both malaria and Pseudomonas aeruginosa infections (Wiesner et al., Curr. Pharm. Des. 2008, 14, 855; Cheng et al., Biochem. Pharmacol. 1973, 22, 3099).
Less is known, however, about the structures and mechanism of action of the last two enzymes in the pathway: IspG and IspH. IspG is E-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP) synthase, EC 1.17.1.1, also known as GcpE. IspH is E-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP) reductase, EC 1.17.1.2, also known as LytB. The penultimate enzyme is IspG, which catalyzes the 2H+/2e− reduction of methylerythritol-cyclo-diphosphate (MEcPP, 1) to HMBPP (2), while the terminal enzyme, IspH, catalyzes the 2H+/2e− reduction of HMBPP (2) to isopentenyl diphosphate (IPP, 3) and dimethylallyl diphosphate (DMAPP, 4) in a ˜5:1 ratio.

The structure of IspG has not yet been reported, while two structures have been published for IspH, one from Aquifex aeolicus (Rekittke et al., J. Am. Chem. Soc. 2008, 130, 17206), and the other from E. coli (Grawert et al., Angew. Chem. Int. Ed. Engl. 2009, 48(31), 5756). According to these reports, both structures contain Fe3S4 clusters. However, these observations are in contrast to the conclusions drawn from both Mössbauer spectroscopy (Xiao et al., J. Am. Chem. Soc. 2009, 131(29), 9931) and EPR spectroscopy (Wolff et al., FEBS Lett. 2003, 541(1-3), 115). Both spectroscopic methods lead to the conclusion that Fe4S4 clusters are responsible for catalysis. The same conclusion was arrived at from the results of microchemical analyses. It therefore seems possible that the Fe4S4 cluster, while catalytically active, may be relatively labile, as found, for example, in aconitase and in pyruvate-formation lyase activating factor. To date, there has been only one report of an IspH inhibitor (Van Hoof, J. Org. Chem. 2008, 73, 1365), which provided an IC50 value of ˜1-2 mM.
Accordingly, newly identified inhibitors of isoprenoid biosynthesis enzymes are needed to further study the Rohmer pathway. New compounds are also needed to develop effective therapeutic methods, including methods to inhibit the activity of isoprenoid biosynthesis enzymes, and methods for treating diseases such as malaria and other infections, due to increased resistance to currently known antibiotics.